Current options for alleviating pain due to vertebral fracture include vertebroplasty and kyphoplasty, which uses balloons inserted into the vertebral space to expand and compress the trabecular bone tissue to create a central cavity within the vertebra. Following cavity preparation, polymethylmethacrylate (PMMA), an acrylic bone cement, is injected into the cavity using percutaneous techniques. Typically, PMMA bone cement is used to restore the mechanical integrity of the vertebral body by stabilizing the trabecular fractures, thereby relieving pain.
One issue associated with vertebro- or kyphoplasty using PMMA cement includes the increased rigidity of the vertebral body in comparison to the superior and inferior vertebrae due to the modulus of the PMMA (1-3 GPa) as compared to trabecular bone tissue (0.05 GPa). This increased rigidity can lead to stress inconsistencies along the length of the spine and may lead to subsequent fractures. Indeed, subsequent facture has been reported in 26% of kyphoplasty cases. See Fribourg, D., et al., Incidence of subsequent vertebral fracture after kyphoplasty. The Spine Journal, 2003. 3(95S).
Moreover, while not frequently observed, pulmonary embolism leading to cardiac failure has been reported in patients receiving acrylic-based vertebroplasty, causing concern when the PMMA exits its intended locale. Hillmeier, J., P.-J. Meeder, and H. C. Kasperlk, The Spine Journal, 2003. 3(117S). Additionally, PMMA does not offer the advantage of osteoconductivity that is appreciated with calcium phosphate-based cements.
Because of the non-optimal properties associated with PMMA cements, certain alternative cementing materials have been investigated. Numerous groups have examined bioactive cements, either calcium phosphate cements or polymeric cements containing bioactive ceramics. While the bioactivity of these materials has been accepted as an improvement over PMMA, the mechanical properties of these cements have been questioned for sufficient fatigue strength and similarly high modulus mismatches to cancellous bone.
Recently, injectable bone substitutes combining polymers and bioactive ceramics have been described. One cement incorporated various bioactive glass beads and calcium phosphate granules to reinforce the polymer. While these composites provided better mechanical properties and higher bioactivity, some of the bioactive beads separated from the bone cement, leading to poor cement-bone interfacial properties. Lack of adhesion between the ceramic fillers and the polymer matrix is the major factor responsible for the filler-matrix debonding, ultimately resulting in a lower load carrying capacity of the cement. ((a) Kokubo, T., H.-M. Kim, and M. Kawashita, Biomaterials, 2003. 24(13): p. 2161-2175 (b) Rea, S. M., et al. Biomaterials, 2004. 25(18): p. 4503-4512 (c) Juhasz, J. A., et al. Biomaterials, 2004. 25(6): p. 949-955 (d) Kamitakahara, M., et al. Biomaterials, 2001. 22(23): p 3191-3196 (e) Ren, L., et al. Journal of Non-Crystalline Solids, 2001. 285(1-3): p. 116-122 (f) Kokubo, T., Acta Materialia, 1998. 46(7): p. 2519-2527).
Thus, there remains a need for injectable, bioactive materials having a compression modulus similar to that of vertebral trabecular bone tissue and having suitable material-bone interfacial properties.